This invention relates to an image deleting agent for offset printing plates which use silver images produced on the offset printing plates as ink receptive areas which deletes the ink receptivity of the silver images by rendering hydrophilic the undesired portion of the silver images. More particularly, it relates to removal of ink receptivity of undesired portion of silver images photographically or physically formed on offset printing plates by rendering completely hydrophilic that portion of the silver images without giving any adverse effect on printing characteristics and properties of substrate by treating the surface with a specific treating agents at an optional stage of printing procedure. Usually, such treatment is called "deletion" in the field of printing and so the inventors use this terminology in this specification. The treating agent used for deletion is called "deleting agent".
Generally, portions of printing plate made by photomechanical process which require deletion include those which are already present in originals to be copied, produced at the time of exposure or produced due to uneven developing treatment, etc.
That is, for example, there are cases where the originals contain undesired images or have dusts, dirty powders, finger prints and other stains and furthermore, when laminated originals are used, a shadow may be produced by the exposure. In order to obtain good printed matters, said portions must be deleted at the stages of plate making and printing. Generally, the deletion is an essential treatment and so it is preferred that necessary deletion can be carried out at an optional stages in plate making and printing procedures.
Many proposals have been made for deleting unnecessary images of offset printing plates which use silver images as ink receptive areas. For example, there are the following methods, namely, a method which comprises precipitating a hydrophilic metal salt, rubber, latex, gelatin or other hydrophilic polymer or pigment on the silver image; a method which comprises mechanically removing the silver image by erasers, knives, etc.; a method which comprises removing metallic silver of the image areas by treating it with a bleaching-fixing solution for silver images which contains an oxidant such as a bichromate or ferricyanide and a sulfite or thiosulfate; etc. However, it is difficult according to these methods to completely accomplish the deletion. That is, these methods have the following defects, namely, oleophilic images are exposed again during printing, stains and other problems may occur, the treatment may require much time to reduce working efficiency, stability of the deletion fluid is not sufficient causing reduction of deleting ability during storage. Furthermore, it is actually difficult to detele images after printing has been once started, namely, an ink film has been formed on silver images.
The deleting agent disclosed in Japanese Patent Application "Kokai" (Laid-open) No. 92101/73 which comprises a compound having mercapto or thion group and a hydrophilic group (e.g., hydroxyl group, carboxyl group, sulfonic group, etc.) in the same molecule and which further improved deleting agents disclosed in Japanese Patent Application "Kokai" (Laid-open), No. 21901/76 and No. 53002/79 have considerably solved the conventional defects and can make hydrophilic the silver images by almost instantaneous reaction.
However, although these deleting agents have excellent deleting ability, they still have the following defects, namely, they have bad odor peculiar to mercapto compounds, they are easily oxidized like cystine and are sometimes unstable when used as deletion fluid, they have adverse effects on human bodies because they are used as strongly acidic solutions and thus handleability and operability decrease.
Therefore, development of odorless deleting agents which satisfy the conditions required for deleting agents as mentioned above have been strongly demanded.